The Centers for Disease Control reported that within the U.S. alone 109,167 cases of Acquired Immunodeficiency Disease Syndrome (AIDS) have been diagnosed since 1981 and that approximately 40,000 persons are living with AIDS. These numbers, however, represent individuals who are currently showing the signs and symptoms of this progressive disease and not the estimated 1.5 million Americans who are infected by the Human Immunodeficiency Virus (HIV) (Grimes et al, 1990). Originally, the HIV infection/AIDS epidemic in the United States was confined to the homosexual population and intravenous drug users. However, the infection has now proceeded into the domain of the American family through heterosexual transmission and vertical transmission. In fact, from the latest figures, the population which has had the greatest increase in sero-positivity for the virus has been women between the ages of 18-24 years of age (Sato et al, 1989). This move into the heterosexual population can also be seen in a recent study that assesses the effect of HIV on the mortality of women in the United States between 15 to 44 years of age. Between 1985 and 1988, the death rate for HIV/AIDS quadrupled (0.6 per 100,000 to 2.5 per 100,000), and by 1987, HIV/AIDS had become one of the 10 leading causes of death within this population. The majority of deaths in both black and white women occurred in women 25 to 35 years of age, for whom HIV-related deaths accounted for 11% and 3% of all deaths in 1988, respectively. If current mortality trends continue, HIV/AIDS can be expected to become one of the five leading causes of death in women of reproductive age by the mid 1990's. Because women infected with HIV are the major source of infection for infants, these trends in AIDS mortality in women forecast the impact of HIV on mortality in children as well (Chu et al, 1990).
With the exception of the intravenous drug use, the most common modality of transmission of HIV virus is by sexual activity. With this mode of transmission, the HIV virus' first contact with a naive individual is at the mucosal surface, be it the urogenital tract, gastrointestinal tract or the oral cavity. Once the virus comes in contact with these surfaces, it is thought that the virus enters the body through breaches in the epithelial surface such as small cuts or abrasions, or possibly directly through the epithelial cells. Directly underlying the epithelial cells is a space composed of loose network of connective tissue and lymph call the lamina propria containing the body's second line of defense to invading organisms. This fluid-filled space contains immunologically reactive cells in the form of macrophages, NK cells, Langerhans cells as well as T and B lymphocytes. In addition, the fluid from this space is continually being drained into regional lymph nodes containing more of these defensive cells. It is within the lamina propria that HIV first comes in contact with immune cells.
Like all viruses, the HIV viral particle only consists of genetic material enclosed in a protective capsule and relies totally on the machinery of other cells for replication and production of progeny. More specifically, the virus contains a protein on its surface which functions to bind the virus to a potential host cell. However, once bound, the virus proceeds through various means to get its genes into the host cell. Information contained within these nucleic acids instructs the host's cells biochemical equipment to preferentially process the viral information over that of the host cells' own products. Within the initial processing steps, viral proteins are made which in turn are responsible for reproducing numerous copies of the original viral genome. This greatly amplifies these specific nucleic acids over that contained in the original host cell. In turn, this newly synthesized genetic material, which is now in great abundance, increases the utilization of the host cell for production of the encapsulating material in which the viral genes are finally packaged and released to begin the infectious cycle once more. The virus' parasitic use of these host cells is quite abusive and depletes it of its energy and other necessary resources. Thus, such an infection is lethal for the host cell.
The CD4 antigen is the ligand used by the virus to enter human cells and begin the replication cycle which produces an endless cycle of more infective viral particles. In a non-infected host, the most abundant cells in the lamina propria which contain the CD4 antigen are the Langerhans cells. These cells are similar to macrophages in many respects but do not migrate from place to place as do macrophages. It is the Langerhans cells that are first to become infected and form the reservoir of shedding viruses which infect the remaining cells in the body that bear the CD4 antigen. However, because Langerhans cells are stationary, rarely do they enter the systemic system. Thus, for HIV viral particles to gain access to the blood, they must first infect other CD4-bearing cells which do migrate systemically. Most likely, these cells are lymphocytes in transit through the local lymphatic system. The shedding virus infect these cells and then through the natural trafficking mechanisms of the lymphocytes finally enter the blood stream. However, the course of these events occur over long periods of time ranging from days to months. This is the latency period which has recently been of such controversy. Early reports suggested that the HIV virus may remain dormant within cells for several months before proceeding to shed. However, current data give evidence that such a dormant period does not exist and the virus begins to shed immediately after infection.
Although the precise mechanisms of how immunoglobulins neutralize virus is not known, IgG, IgA and IgM antibodies have been described which are capable of neutralizing viral particles. Most of the vaccines directed at preventing HIV infections using either attenuated viral particles or recombinant HIV-1 envelope or core proteins have been administered intramuscularly or subcutaneously into individuals. This mode of vaccination is directed at humoral or systemic immune responses and elicits the production of IgG antibodies. This type of antibody remains primarily in the bloodstream of the individual in order to prevent bloodborn disease. It has been suggested that some of these IgG antibodies that are reactive with the HIV virus may possibly enhance HIV-1 infection even though they had previously been shown to be neutralizing. It is thought that this enhancing effect occurs as a result of the IgG antibody acting as a bridge between the virus and the receptors for these immunoglobulins on antibody binding cells (Takeda et al, 1988).
IgA antibody is secreted by differentiated, activated lymphocytes in the lamina propria. The structure and function of IgA in accordance with the major objectives of mucosal immunity, is quite distinct from that of the IgG produced by the systemic immune system. For example, IgA antibodies reach the mucosa much faster and in greater abundance than that of IgG (Halsey, J. F. et al, 1980). The IgA on mucosal surfaces occurs predominantly in dimeric and tetrameric forms, having four to eight antigen-binding sites. Polymeric IgA has been shown to neutralize viruses more effectively than the monomer (Dimmock, 1993; Taylor et al, 1985). In addition, regions on IgA unrelated to the antigen-binding domains are recognized by the mucosal epithelium. This epithelium forms a boundary of cells between the lamina propria and the outside of the body. These cells will attach to areas on the IgA molecule, via receptors, and actively displace these antibodies from the lamina propria to the outside of the body. Thus these antibodies do not rely on mere diffusion to reach the mucosa in contrast to all the other types of antibodies produced in the body.
It is well known that virus-neutralizing antibodies occur at mucosal cell surfaces and are important in preventing local infection and disease (Ogra et al., 1989). The direct inhibitory effect of immunoglobulin A on the adherence of virus and bacteria to host mucosal epithelial cells has been documented in many experimental systems (Wold et al., 1990; Abraham et al., 1985). Both non-specific hydrophobic interactions as well as specific inhibition of binding between bacterial surface adhesions and complementary surface receptors on host epithelial cells are involved in this process (McGhee et al., 1992). It has also been shown that intracellular antibody-antigen reactions between IgA and viruses reduces the ability of virus to replicate (Mazanec et al., 1992).
Another rather unique feature of IgA antibody molecules is their rather extensive glycosylation. The sugar moieties covalently linked to IgA molecules is thought to reduce the proteolysis of the antibodies by proteases which are found in the digestive tract and other mucosal surfaces. Along with this glycosylation, another peptide is added to the IgA as it traffics through the epithelial cell. This peptide, know as the secretory piece or secretory component, further reduces the effects of any possible enzymatic digestion. Thus, the distinctive structure of the IgA molecule renders it resistant to degradation by enzymes and unusually prepared to reside in the milieu of the mucosa as compared with antibodies of other isotypes (e.g., IgG, IgM, etc.) that might be present in the body and on the mucosal surfaces.
Although the encounter of an IgA antibody with its corresponding antigen in the mucosa inhibits the absorption of the antigen to the epithelial cells, it does not result in the activation of the complement system and the generation of the cleavage products of the C3 and C5 components. It has been well documented that an encounter between an IgG antibody on the mucosal cell surface with its corresponding antigen, although inhibiting the absorption of the antigen to the mucosal cells, activates the complement system resulting in local tissue damage and an increased absorption of bystander antigens. Thus, it seems that one of the major biological roles of IgA at mucosal surfaces is the mitigation of the inflammatory side effects brought about by other immune effector mechanisms.
Passive immunity has been known since the science of Immunology began. This type of immunity differs from active immunity in that the immunoglobulins from one animal are transferred into another animal rather than using a vaccine or antigen to induce the animals' own cells to produce an antibody response. Early use of such techniques were used to cure numerous bacterial as well as viral infections. It was determined early on that if the immunoglobulin was produced in one species of animal and injected into a species of animal different from the first, a second injection of the same immunoglobulin preparation would yield devastating effects and most often result in the death of the recipient animal. However, if the antibodies of interest were produced in the same species as the recipient animal, such immunoglobulins could be administered repeatedly without ill effects. Such preparations are currently licensed and used for numerous different purposes. Gamma globulin received before a person travels abroad is one such preparation. The antibodies received within these passive vaccines are predominately IgG with only a minor concentration of IgA.
The most plentiful source of immunoglobulin A comes from lactating mothers in their breast milk. It has been demonstrated that this source of IgA protects the newborn from disease during the first months of life. This natural passive immunity from mother to child has been used since mammals began walking the earth.
It is an object of the subject invention to provide a method for protecting individuals from HIV infection through passive immunity using IgA antibodies to HIV at mucosal surfaces.